The pitot heat provides anti-ice capability by electrically heating the probe with a standard resistance heating element. Heat has no affect on the reading of the airspeed system. It provides about 10 watts per square inch to the tube (about like a 150 watt light bulb would produce, so it can remove the skin from your hand quite easily!)
The usual procedures are to turn it on when in visible moisture (cloud, rain, snow, etc.) when the outside temp is below +4 degrees C or so (the specifics are in the flight manual for the type.)
If not used, the pitot tube gradually blocks, and generally starts to lose indicated airspeed, and will eventually show zero knots.
Sometimes, the static ports on the fuselage (if provided) are heated by the same switch.